The path to quantum mechanics began in 1859 with the work of the German physicist Gustav Kirchoff who studied the spectrum of light emitted by the Sun. The spectrum is continuous with a gently varying intensity along with many sharp, dark lines superimposed. One set of lines, the so-called D-lines were interpreted as coming from the absorption of light from the continuous spectrum emanating from the interior of the Sun by sodium atoms at its surface. To understand the nature of the continuous spectrum, Kirchoff began to consider the emission and absorption of radiation by heated materials in general.
In the mid-1880's the Austrian physicist, Ludwig Boltzmann, derived an expression for the total energy density emitted by an ideal blackbody. The formula, now known as the Stefan-Boltzmann law, stated that the total energy density is proportional to the fourth power of the temperature. Another German physicist, Wilhelm Wien, made great strides in finding the intensity distribution function that would match the spectra of heated materials. The distribution he suggested worked well for high frequencies and gave the correct prediction for the wavelength at which the maximum intensity occurred, but was later found to give wrong predictions in the long wavelength, infrared region.
The revolutionary work, published in 1900, of the German physicist, Max Planck, succeeded in giving an intensity distribution that was in good agreement with all experimental data. In order to derive his result, however, Planck had to admit that classical physics was wrong. He had to make a drastic, quite unjustified assumption: that the oscillators could only emit and absorb energy in discrete units called quanta.
Refer to the following sections in Modern Physics by Bernstein, Fishbane and Gasiorowicz:
Part 2: Historical Introduction, page 101.
Section 4-4: Derivation of blackbody formula, page 116.
Section 12-8: Blackbody radiation and the Bose-Einstein distribution of photons, page 353.
Section 18-4: Cosmic background radiation, page 555.
© 2000 by Prentice-Hall, Inc. A Pearson Company